1. Field of the Invention
The present invention relates to a radar apparatus provided with a switch for selecting any of the signals received by a plurality of antennas respectively, and particularly to a radar apparatus capable of detecting a malfunction of the switch.
2. Description of the Related Art Prior Art
As a method of determining the direction of a target, a phase monopulse system for computing the target from the phase difference between the reflected waves received by two antennas is known.
FIG. 1 is a figure for explaining the phase monopulse system. As shown in FIG. 1, a phase monopulse radar apparatus receives reflected wave from an object by using two receiving antennas to determine the angular direction θ of the target from the phase difference φ between [the both receiving antennas] by using the following formula:θ=sin−1(λφ/2πd0)   (1)
wherein, λ is the wavelength of a radar wave, and d0 is the interval between the antennas.
As an in-car radar, an FM-CW radar is used in which a transmission wave subjected to FM modulation with a triangular wave is used to obtain the distance to and relative speed of an object from the sum and the difference of the frequencies between an ascending section and a descending section of the triangular wave of a beat signal generated by mixing a reception wave with a part of the transmission wave. In this FM-CW radar, the abovementioned reception phase difference φ is computed from a phase value of the peak generated as a result of Fourier transformation of the beat signal.
FIG. 2 is a block diagram showing a configuration of the phase monopulse FM-CW radar apparatus. As shown in FIG. 2, there are disposed three antennas, AT0, AT1 and AT2, and these antennas are preferably disposed at irregular intervals such that, when the wavelength of, for example, a carrier wave is λ, the space between the AT0 and AT1 is 5λ/4 and the space between the AT1 and AT2 is 6λ/4.
A transmission signal, which is outputted from a voltage-controlled oscillator (VCO) 10 and subjected to FM modulation with a triangular wave, is amplified by a transmitting amplifier 14, passes through a circulator 16, and is then transmitted from an antenna. In the radar apparatus shown in FIG. 2, transmission and reception are performed using one of the three antennas, AT0, AT1 and AT2, selected by switches SW0, SW1 and SW2. A reception signal, which is transmitted by the antenna selected by the switches SW0, SW1 and SW2 and received by the antenna selected by the switches SW0, SW1 and SW2, passes through the circulator 16, is amplified by a receiving amplifier 26, and mixed with a part of a transmission wave at a mixer 28, whereby a beat signal is generated. The beat signal generated in the mixer 28 is converted into a digital signal in an A/D converter 32, subjected to fast Fourier transformation (FFT) by a fast Fourier transformation section 34, and then inputted to a CPU 36. The CPU 36 computes a direction, distance to, and relative speed of a target on the basis of a frequency analysis of the beat signal.
When obtaining the reception phase difference φ, two antennas are selected from the three antennas AT0, AT1 and AT2, and, for each peak obtained from a reception signal (reflected wave) of each of the selected antennas, a difference value (phase difference φ) between phases of the corresponding peaks is obtained. For example, a combination of the antenna AT0 and antenna AT1, a combination of the antenna AT0 and antenna AT2, and a combination of the antenna AT1 and antenna AT2 can be taken.
It should be noted that the patent literature 1 (Japanese Patent Application Laid-Open No. H11-205207) discloses a configuration in which, when a predetermined level is not exceeded by the maximum reception level of an antenna which is alternately received, this antenna is judged as abnormal. Furthermore, the patent literature 2 (Japanese Patent Application Laid-Open No. H5-327568) discloses a configuration in which, when the average value of the differences among a plurality of received voltages of antennas is larger than a threshold, the antennas are judged as malfunctioning antennas. The patent literature 3 (Japanese Patent Application Laid-Open No. 2001-127680) discloses a configuration in which the past reception levels received with the same antenna elements are averaged. Moreover, the patent literature 4 (Japanese Patent Application Laid-Open No. H3-52427) listed blow discloses a configuration in which, when two transmission outputs are combined and then outputted because of a malfunction of a diode switch, only the beat frequency of the combined output is detected to detect a malfunction of the diode switch.
On the other hand, in the configuration shown in FIG. 2, since there is one transmitting/receiving path with respect to three antennas, transmission and reception needs to be performed using one antenna in a time division manner. Therefore, one antenna which performs transmission and reception is selected by a switching control of the switches SW0, SW1 and SW2 at each measuring timing.
FIG. 3 is a figure showing a time chart of control signals of the switches for switching transmission and reception of the antennas. In the example shown in FIG. 3, the switching control performed by the switches is repeatedly performed at the subsequent timings t1 to t6 in response to a switching of transmission and reception.
t1: Transmission performed by the antenna AT0
t2: Reception performed by the antenna AT0
t3: Transmission performed by the antenna AT0
t4: Reception performed by the antenna AT1
t5: Transmission performed by the antenna AT0
t6: Reception performed by the antenna AT2
In the case in which the switches are not turned ON/OFF normally due to a malfunction of the switches in such switching control performed by the switches, measurements cannot be carried out accurately. For example, at the timing t2 at which reception is performed by the antenna AT0, the switches SW1 and SW2 of the antennas AT1 and AT2 need to be OFF, but if the antenna AT1 is ON because of a malfunction of the SW1, a beat signal based on a composite signal of signals received by both antenna AT0 and antenna AT1 is generated. As described above, if a switch is not turned OFF when it is supposed to be turned OFF, or if [the switch] is not turned ON when it is supposed to be turned ON because of a malfunction of the switch, measurements cannot be carried out accurately.